You just need a healthy one! However, your tank should be at least 2.5 US gallons with a filter and heater. If you are getting a betta from a pet store, you should look for:
clear, unswollen eyes
smooth scales with no white dots(ich)
no cottony/fungus stuff on him
fins intact with no black rims(fin rot)
he should flare if you hold up a mirror or if he sees another betta
hope this helps
...( o(o)~ I really like bananas!